My honest opinion: I don't believe "Bruce is cursed to be Batman forever" was a key message of TDK. It was merely one possible direction the story could go if it continued.

I think the message of TDK is more about having to make impossible choices that may otherwise be perceived as wrong in order to defeat a greater evil in a time of crisis. It's about coming face to face with the devil and having to make serious compromises in order to come out with some measure of victory. And I think it's about accepting that it might have been your own actions that invited the devil to come wreak havoc in the first place.

I don't think any of that has any bearing how how long Bruce is supposed to be Batman for. TDK did indeed sum up some of the classic aspects of the Batman mythos, but I believe it did so in a way that works in a confined way. The story was free to go any number of directions, as the ending had a large degree of ambivalence to it.